It has heretofore been proposed to measure concentration of solid materials, such as dust, powder or granular products, flowing in a pneumatic conveyor line. One typical installation would be downstream of a dust collector, with the device being configured to sense material in suspension and thus indicating failure at the dust collector. Another application is as a flow sensor to monitor continuous flow of material in a pneumatic conveyor, and to provide an indication of flow termination due, for example, to a plugged conveyor or loss of material feed. It is a general object of the present invention to provide a method and apparatus for measuring concentration of solid particles in a fluid stream employing the triboelectric effect that are of simple and inexpensive design, that do not require recalibration during operation, that automatically adjust for voltage and temperature drift during operation, that have operator selectable sensitivity, and that may be readily employed with minimum operator configuration as either a flow sensor or flow-interruption sensor.
Briefly stated, concentration of solid particles in a fluid stream is monitored in accordance with a presently preferred implementation of the invention by positioning a sensor or probe in the fluid stream so as to provide a sensor output signal that varies as a function of triboelectric effect of solid particles in the stream on the probe, and thus as a function of concentration of the particles in the fluid stream. In accordance with a distinguishing feature of the present invention, both magnitude and rate of change of the sensor signal are monitored, and an output signal is provided when both magnitude and rate of change of the sensor signal exceed a preselected threshold. By monitoring both magnitude and rate of change of the sensor signal, gradual changes due to voltage and temperature drift are ignored. This technique eliminates any requirement for a dead band to accommodate voltage drift, or for additional circuitry to adjust for temperature compensation. Following selection of desired sensitivity and installation of the apparatus, no further calibration is necessary. Inexpensive off-the-shelf electronic components can be utilized without batch testing for low offset voltage or temperature drift.
Apparatus for measuring concentration of solid particles in a fluid stream in accordance with the present invention thus includes a probe for disposition in the fluid stream to provide an electrical probe signal as a triboelectric function of concentration of solid particles in the fluid stream. A first circuit is coupled to the probe for providing a first electrical signal as a proportional function of magnitude of the probe signal. A second circuit is coupled to the probe for providing a second electrical signal as a function of rate of change of the probe signal. Differential amplifiers received the first and second signals, and provide a third signal as a function of a difference therebetween. An apparatus output signal is provided when such difference exceeds a preselected threshold. The first circuit in the preferred embodiment of the invention includes facility for adjusting sensitivity of the apparatus by selectively proportionately scaling the first signal to the probe signal. This sensitivity adjustment circuit comprises a switch-selectable voltage divider. The differential amplifier provides the third signal when either of the first and second signals exceeds the other by more than the preselected threshold. In this way, the apparatus is responsive to both positive and negative changes in concentration of solid particles in the fluid stream, and thus may be employed as either a flow sensor or a flow-interruption sensor. In this respect, the apparatus circuitry also includes facility for selectively delaying provision of the apparatus output signal, and fail-safe facility for providing the apparatus output signal independent of particle flow in the event of circuit or power failure.